Effects of exogenous melatonin on photosynthesis physiology biochemistry of blue honeysuckle seedlings under saline-alkali stress

Salt and alkali stress severely restricts the growth, development, and cultivation promotion of blue honeysuckle ( Lonicera caerulea L.). Exogenous melatonin (MT) plays a significant role in plant stress resistance regulation, but its alleviation mechanism under salt and alkali stress for blue honeysuckle remains to be systematically clarified. In order to explore the regulatory effect of exogenous melatonin on saline-alkali tolerance, this study used blue honeysuckle seedlings as materials to analyze the effects of different concentrations of melatonin (0, 25, 100, 200 µmol/L) on photosynthesis, physiological and biochemical characteristics, and gene expression of seedlings under saline-alkali stress. Results showed 100 µmol/L MT exhibited optimal effects: it mitigated stress-induced photoinhibition, elevated PSII’s maximum photochemical efficiency (Fv/Fm), and optimized light energy distribution (ABS/RC reduced); upregulated SOD, POD, CAT activities, lowering MDA content to inhibit membrane lipid peroxidation; regulated soluble sugar and proline accumulation, with proline content significantly higher to maintain osmotic balance. Transcriptome analysis revealed it activated stress response networks by upregulating NAC/WRKY transcription factors and auxin/ABA-related genes. Notably, 200 µmol/L MT attenuated mitigation via oxidative feedback inhibition. In conclusion, 100 µmol/L exogenous melatonin significantly improved the saline-alkali tolerance of blue honeysuckle seedlings by synergistically enhancing photosynthetic stability, antioxidant defense, and transcriptional reprogramming, which provides a theoretical basis for crop cultivation in saline-alkali land and agricultural application of melatonin.